The folding angle distribution of stretched and negatively supercoiled DNA double-helix is investigated based on a previously proposed model of double-stranded biopolymers [H. Zhou et al., Phys. Rev. Lett. 82, 4560 (1999)]. It is shown that pulling can cause transition of a negatively supercoiled DNA double-helix from the right-handed B-form to a left-handed configuration which resembles DNA Z-form in some important aspects. The energetics of this possible transition is calculated and a comparison with recent experimental observations is qualitatively discussed.